A light-emitting device of a chip-on-board (COB) type is used as a light source of an illumination device such as an LED lamp. A light-emitting device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2008-277561 is provided with a substrate, a plurality of light-emitting diode columns, reflector, and sealing member.
The substrate includes a mount surface on which a plurality of power-supply conductors are arranged. The light-emitting diode columns linearly extend along the mount surface of the substrate, and are arranged at intervals in a direction perpendicular to a direction in which the light-emitting diode columns extend.
Each of the light-emitting diode columns is provided with a plurality of light-emitting diode chips configured to emit blue light. The light-emitting diode chips are arranged at intervals in a line. Each of the light-emitting diode chips includes a positive electrode, and negative electrode, and is joined to the mount surface of the substrate with a transparent adhesive.
Light-emitting diode chips adjacent to each other in the direction in which the light-emitting diode column extends are electrically connected to each other through a first bonding wire. An end of the first bonding wire is connected to a positive electrode of one of adjacent light-emitting diode chips. The other end of the first bonding wire is connected to a negative electrode of the other light-emitting diode chip. Accordingly, the plurality of light-emitting diode chips constituting a light-emitting diode column are connected in series. Furthermore, each of light-emitting diode chips positioned at both ends of a light-emitting diode column is electrically connected to a power-supply conductor of the substrate through a second bonding wire.
The reflector is joined to the mount surface of the substrate to surround the light-emitting diode columns. The sealing member is constituted of a transparent silicon resin mixed with a yellow fluorescent material. The sealing member is filled into an area surrounded by the reflector, and seals the light-emitting diode columns on the mount surface.
According to such a light-emitting device, the light-emitting diode chips emit light all at once by energization. The blue light emitted from the light-emitting diode chips is made incident on the sealing member. Thereby, the yellow fluorescent material is excited to emit yellow light, which is a complementary color of blue. The yellow light and blue light are mixed with each other to produce white light, and the white light is used for the illumination purpose.
According to the light-emitting device disclosed in the aforementioned Jpn. Pat. Appln. KOKAI Publication No. 2008-277561, the substrate is constituted of white epoxy resin in order that the light emitted from the light-emitting diode chips may be efficiently extracted. In order to obtain a white substrate, normally powder of titanium oxide is mixed into the epoxy resin.
However, titanium oxide is liable to be decomposed by blue light when blue light emitted from the light-emitting diode chips is made incident on the sealing member. Accordingly, the mount surface of the substrate is gradually deteriorated to be brought into a roughened state. Accordingly, it becomes difficult to maintain the light reflection performance required of the white mount surface for a long period.
As a countermeasure against the above, it has been conventionally tried to form a plurality of metallic light reflection layers on the mount surface of the substrate. The light reflection layers are used to reflect light emitted from the light-emitting diode chips in the direction toward the substrate in a direction in which light should be extracted. The light reflection layers are arranged at intervals so that they may correspond to positions of the light-emitting diode chips. In other words, a plurality of light-emitting diode chips are individually mounted on the light reflection layer.
However, it has been made clear even in the light-emitting device using the light reflection layers that there is the following problem to be solved. That is, the light reflection layers are arranged at intervals on the mount surface of the substrate, and hence the mount surface formed of the epoxy resin is exposed at parts between adjacent light reflection layers. Accordingly, it is inevitable that light emitted from the light-emitting diode chips is shined into the mount surface at parts between adjacent light reflection layers. When the epoxy resin receives light emitted from the light-emitting diode chips, part of the resin components (organic substance) constituting the skeleton of the epoxy resin are decomposed by the light to be gasified. It is conceivable that this phenomenon is caused by decomposition of uncombined molecules contained in the epoxy resin by the light.
Furthermore, the light reflection layers are covered with the sealing member together with the light-emitting diode chips. The silicon resin constituting the sealing member has permeability to gas. Accordingly, it cannot be denied that a gas released from the epoxy resin constituting the substrate passes through the sealing member to reach the light reflection layers.
When the surface of the light reflection layer receiving heat of the light-emitting diode chips is exposed to the gas, the light reflection layer reacts with the gas, and the surface of the light reflection layer is discolored to be darkened. It has been made clear that such a phenomenon is conspicuous when the surface of the light reflection layer is constituted of silver. The phenomenon in which the surface of the light reflection layer is discolored to be darkened lasts until the release of the gas stops.
When the surface of the light reflection layer is discolored dark, the light reflection performance of the light reflection layer is deteriorated. Accordingly, although the light reflection layers are provided on the mount surface of the substrate, it becomes impossible to efficiently extract light. As a result, it becomes difficult to maintain the expected luminous flux required of the light-emitting device for a long period.
From the above description, in the light-emitting device in the current state, there is yet room for improvement from the viewpoint of efficiently extracting light emitted from the light-emitting diode chips, and maintaining the desired luminous flux for a long time.